As decreed by Pope Gregory XIII, October 4, 1582, was followed by October 15, 1582. Thus ended the 1600-year
reign of the Julian calendar upon which the Gregorian calendar is based, and thus began the calendar which
DECwindows Calendar uses to measure time.

Calendars based on sun and moon movement were used even by the ancients, but the first reasonably accurate one
was the 365 1/4-day cycle calculated by the Greek Sosigenes. This was the calendar authorized by Julius
Caesar in 46 BC. The Julian calendar (not to be confused with the Julian period; see below) had 3 years
of 365 days each, followed by a fourth year of 366 days.

The 365 1/4-day cycle was more accurately defined in 730 AD by the Venerable Bede, an Anglo-Saxon monk, who
shortened the time by 11 minutes, 14 seconds. This accumulates to a whole day's error every 128 years, or
a little more than 3 days every 400 years. This being the Dark Ages, nothing was done to adjust the Calendar,
despite Roger Bacon sending a note to Pope Clement IV, informing him of the drifting of the date for the
vernal equinox. Later, Pope Sixtus IV did become convinced that another reform was needed and called the
German astronomer Regiomontanus to Rome to advise him. Unfortunately, Regiomontanus died of the plague shortly
thereafter and the plans died with him.

Thursday, October 4, 1582 was the next time the calendar was adjusted. This last day of the Julian
calendar was followed by Friday, October 15. So began the Gregorian calendar that we use today, named after
Pope Gregory XIII. He commissioned the mathematician Father Christopher Clavius, S.J., to do the necessary
calculations, having been authorized to reform the calendar by the Council of Trent in 1545.

The Vatican librarian Aloysius Giglio provided a formula for long-range accuracy. He suggested that
every fourth year be a leap year, except for century years that are not divisible by 400. Thus 1700, 1800,
and 1900 would not be leap years, but 2000 would be, because 2000 is divisible by 400. This rule eliminated
3 leap years every 4 centuries, making the calendar sufficiently correct for most ordinary purposes.

Political Acceptance in Europe

Italy, Portugal, and Luxembourg. By 1584, Belgium, parts of the Netherlands, Switzerland, and most Catholic
German states had joined, and by 1587, so had Hungary. It was not until 1699-1700 that these countries were
joined by the rest of the Netherlands, Denmark, and the Protestant German countries.

By the time the British imposed the calendar on all its possessions, in 1752, 11 days needed to be lost.
September 2, 1752, was thus decreed to be followed by September 14. In addition, New Year's day was moved
back from March 25 to January 1. (For example, before, March 24, 1700 had been followed by March 25, 1701).
Among other repercussions, this moved Washington's birth date from February 11, 1731, to February 22,
1732. The following year, 1753, Sweden too adopted the calendar.

In 1793, the French Revolutionary government adopted a calendar of 12 months of 30 days each, with 5 extra
days in September (6 on leap years). The Gregorian calendar was reinstated in 1806 by Napoleon.

Political Acceptance World Wide

Adoption of the calendar in countries outside Europe and its Crown possessions occurred much later, and
often in conjunction with political upheaval: Japan in 1873, Egypt in 1875, China in 1912, and Turkey in 1917.

In 1918, Russia's revolutionary government decreed that January 31, 1918, would be followed by February 14, 1918.

Religious Acceptance Worldwide

German Protestants used the old calendar until 1776, three quarters of a century after their countries had
adopted the Gregorian system.

Sweden retained the old Easter rules for 90 years after switching to the Gregorian calendar, and many Middle
Eastern Christian sects still retain the Julian calendar.

The Russian Orthodox Church still follows the Julian system.

The Julian Period

Astronomers use the Julian period because it is convenient to express long time intervals in days
rather than months, weeks and years. It was devised by Joseph Scaliger, in 1582, who named it after his father
Julius, thus creating the confusion between the Julian (Caesar) calendar and the Julian (Scaliger) period.

Julian Day 1 began at 12:00 noon, January 1, 4713 BC. This date was thought by some to correspond
approximately to the beginning of the universe. Certainly it predated any known astronomical events
known in the 16th century without resorting to negative times. Scaliger decided on the actual date on the
grounds that it was the most recent coincidence of three major chronological cycles:

- The 28-year solar cycle, after which dates in the Julian calendar (for example September 27) return to the same days
of the week (for example Tuesday).

- The 19-year lunar cycle, after which phases of the moon return to the same dates of the year.

- The 15-year indiction cycle, used in ancient Rome for tax regulation.

It takes 7980 years to complete the cycle. Noon of January 1, 1988, marks the beginning of Julian Day 2447161.

The Julian period is also of interest because of its use as a time base by the VMS operating system.

VMS and the Julian Period or:
Why VMS regards November 17, 1858,
as the beginning of time...

The modified Julian date adopted by SAO (Smithsonian Astrophysical Observatory) for satellite tracking is Julian Day 2400000, which turns out to be November 17, 1858.

SAO started tracking satellites with an 8K (nonvirtual) 36-bit IBM 704 in 1957, when Sputnik went into orbit. The Julian day was 2435839 on January 1, 1957. This is 11225377 octal, which was too big to fit into an 18-bit field. With only 8K of memory, the 14 bits left over by keeping the Julian date in its own 36-bit word would have been wasted. They also needed the fraction of the current day (for which 18 bits gave enough accuracy), so it was decided to keep the number of days in the left 18 bits and the fraction of a day in the right 18 bits of one word.

Eighteen bits allows the truncated Julian day (the SAO day) to grow as large as 262143, which from November 17, 1858, allowed for 7 centuries. Possibly, the date could only grow as large as 131071 (using 17 bits), but this still covers 3 centuries and leaves the possibility of representing negative time. The 1858 date preceded the oldest star catalogue in use at SAO, which also avoided having to use negative time in any of the satellite tracking calculations.

Ultrix (Unix) Time Origins

The beginning of time for Ultrix systems is:

Thursday January 1 00:00:00 1970

The reason for this date being chosen is that this was the year that UNIX, the "father" of Ultrix, was firstreleased.

Thus dates prior to 1970 are BU; 1970 and later dates are AU.

History and DECwindows Calendar

If you read the topics concerning the political and religious acceptance of the Gregorian calendar, you will see that there is a problem: there are many dates for the conversion from the Julian to the Gregorian
calendar. For example, the British (and therefore the Americans) converted in September 1752, so British and
American Calendar users might expect Calendar to show September 2, 1752, to be followed by September 14,
1752. However, a Russian user would expect to see this jump between January 31, 1918, and February 14, 1918.

DECwindows Calendar conforms to the date of the original decree, therefore no days have been lost since
Friday, October 15, 1582, nor since the beginning of DECwindows Calendar time: January 1, 1600. Thus for
everyone except for users from the majority of the Catholic European countries, which all converted before
this date, there will appear to be an "error" in Calendar, where the conversion actually took place.
This generalization was felt to be acceptable for an application not specifically designed for historians.